1. Technical Field
The present disclosure relates to a power amplifier circuit.
2. Description of the Related Art
Availability of allocatable frequency bands is increasingly tight as radio technology, such as communications and radars, is in widespread use. To improve frequency availability, the use of high frequency bands, such as millimeter wavebands, has started. For example, a 60 GHz band has been used in the field of millimeter-wave communications and a 79 GHz band has been used in the field of millimeter-wave radars.
To ease the tight frequency availability, the use of a frequency band even higher than 100 GHz is expected. Radio apparatuses using a frequency band above 100 GHz are able to use a wider frequency range than the millimeter-wave radio apparatuses using the 60 GHz band or 79 GHz band, and may achieve high-speed communication and implement a high-resolution radar.
Radio integrated circuits (ICs) operating on a frequency above 100 GHz are expected to be manufactured at low costs as a frequency band above 100 GHz is going to be used.
Typically, radio ICs are manufactured of semiconductor through a complementary metal-oxide semiconductor (CMOS) process. The CMOS process allows a low-cost and highly integrated radio IC to be manufactured. Furthermore, through a miniaturization CMOS process, a transistor (radio IC) that is theoretically operable on a frequency above 100 GHz may be manufactured.
When a radio IC operating on a frequency above 100 GHz is manufactured through a miniaturization CMOS process, a margin permitted in designing a power amplifier circuit that is an element of the radio IC is small. High-gain techniques for the power amplifier circuit are being developed in order to operate a radio IC stably despite variations occurring in process, voltage, and temperature (PVT).
A maximum available gain (MAG) is contemplated as a parameter increasing a design margin in the power amplifier circuit. MAG of the power amplifier circuit is decreased by the effect of a parasitic element, such as a parasitic capacitance of a transistor.
Japanese Patent No. 5228017 discloses a power amplifier circuit that increases MAG. The power amplifier circuit neutralizes the effect of a parasitic capacitance Cgd between the gate and drain of a transistor.
Factors that decrease MAG of the power amplifier circuit includes, in addition to the parasitic capacitance Cgd, a parasitic capacitance Cgs between the gate and source of the transistor, and a parasitic capacitance Cds between the drain and source of the transistor. The parasitic capacitance Cgs and the parasitic capacitance Cds are created because the source of the transistor is not grounded by the effect of an inductance parasitic to the source of the transistor.
In the power amplifier circuit disclosed in Japanese Patent No. 5228017, the parasitic elements other than the parasitic capacitance Cgd are not sufficiently neutralized. There is still room for improvement in the decrease of MAG. Particularly, in a high frequency band near a maximum frequency (fmax) of the transistor, the effect of the parasitic elements is pronounced, and MAG of the power amplifier circuit decreases even more.